Sandberg told the crowd: "I hate to be the bearer of bad news but in consumer technology, if you want to know what people like us will do tomorrow, you look at what teenagers are doing today.

"And the latest figures say that only 11 percent of teenagers e-mail daily. So e-mail — I can't imagine life without it — is probably going away. So what do teenagers do? They SMS and increasingly, they use social networking."

Sandberg didn't cite any more in-depth figures or research on the matter, but the fact that she is COO of Facebook, who presumably know more than most companies how teenagers operate online, is certainly reason enough to take her words seriously.